Carbon is known to have four unique crystalline structures, including diamond, graphite, fullerene and carbon nano-tubes. The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall, which can be conceptually obtained by rolling up a graphene sheet or several graphene sheets to form a concentric hollow structure. A graphene sheet is composed of carbon atoms occupying a two-dimensional hexagonal lattice. Carbon nano-tubes have a diameter on the order of a few nanometers to a few hundred nanometers. Carbon nano-tubes can function as either a conductor or a semiconductor, depending on the rolled shape and the diameter of the tubes. Its longitudinal, hollow structure imparts unique mechanical, electrical and chemical properties to the material. Carbon nano-tubes are believed to have great potential for use in field emission devices, hydrogen fuel storage, rechargeable battery electrodes, and composite reinforcements.
However, CNTs are extremely expensive due to the low yield and low production and purification rates commonly associated with all of the current CNT preparation processes. The high material costs have significantly hindered the widespread application of CNTs. Rather than trying to discover much lower-cost processes for nano-tubes, we have worked diligently to develop alternative nano-scaled carbon materials that exhibit comparable properties, but can be produced in larger quantities and at much lower costs. This development work has led to the discovery of processes for producing individual nano-scaled graphite planes (individual graphene sheets) and stacks of multiple nano-scaled graphene sheets, which are collectively called “nano-scaled graphene plates (NGPs).” NGPs could provide unique opportunities for solid state scientists to study the structures and properties of nano carbon materials. The structures of these materials may be best visualized by making a longitudinal scission on the single-wall or multi-wall of a nano-tube along its tube axis direction and then flattening up the resulting sheet or plate. Studies on the structure-property relationship in isolated NGPs could provide insight into the properties of a fullerene structure or nano-tube. Furthermore, these nano materials could potentially become cost-effective substitutes for carbon nano-tubes or other types of nano-rods for various scientific and engineering applications.
Direct synthesis of the NGP material had not been possible, although the material had been conceptually conceived and theoretically predicted to be capable of exhibiting many novel and useful properties. Jang and Huang have provided an indirect synthesis approach for preparing NGPs and related materials [B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006)]. Another process developed by B. Z. Jang, et al. [“Process for Producing Nano-scaled Graphene Plates,” U.S. patent pending, Ser. No. 10/858,814 (Jun. 3, 2004), now abandoned] involves (1) providing a graphite powder containing fine graphite particles (particulates, short fiber segments, carbon whisker, graphitic nano-fibers, or combinations thereof) preferably with at least one dimension smaller than 200 μm (most preferably smaller than 1 μm); (2) exfoliating the graphite crystallites in these particles in such a manner that at least two graphene planes are either partially or fully separated from each other, and (3) mechanical attrition (e.g., ball milling) of the exfoliated particles to become nano-scaled to obtain NGPs. The starting powder type and size, exfoliation conditions (e.g., intercalation chemical type and concentration, temperature cycles, and the mechanical attrition conditions (e.g., ball milling time and intensity) can be varied to generate, by design, various NGP materials with a wide range of graphene plate thickness, width and length values. Ball milling is known to be an effective process for mass-producing ultra-fine powder particles. The processing ease and the wide property ranges that can be achieved with NGP materials make them promising candidates for many important industrial applications. The electronic, thermal and mechanical properties of NGP materials are expected to be comparable to those of carbon nano-tubes; but NGP will be available at much lower costs and in larger quantities.
In this and other methods for making separated graphene or other non-carbon inorganic platelets, the process begins with intercalating lamellar flake particles with an expandable intercalation compound (intercalant), followed by expanding the intercalant to exfoliate the flake particles. Conventional intercalation methods and recent attempts to produce exfoliated products or separated platelets are given in the following representative references:    1. J. W. Kraus, et al., “Preparation of Vermiculite Paper,” U.S. Pat. No. 3,434,917 (Mar. 25, 1969.    2. L. C. Olsen, et al., “Process for Expanding Pyrolytic Graphite,” U.S. Pat. No. 3,885,007 (May 20, 1975).    3. A. Hirschvogel, et al., “Method for the Production of Graphite-Hydrogensulfate,” U.S. Pat. No. 4,091,083 (May 23, 1978).    4. T. Kondo, et al., “Process for Producing Flexible Graphite Product,” U.S. Pat. No. 4,244,934 (Jan. 13, 1981).    5. R. A. Greinke, et al., “Intercalation of Graphite,” U.S. Pat. No. 4,895,713 (Jan. 23, 1990).    6. F. Kang, “Method of Manufacturing Flexible Graphite,” U.S. Pat. No. 5,503,717 (Apr. 2, 1996).    7. F. Kang, “Formic Acid-Graphite Intercalation Compound,” U.S. Pat. No. 5,698,088 (Dec. 16, 1997).    8. P. L. Zaleski, et al. “Method for Expanding Lamellar Forms of Graphite and Resultant Product,” U.S. Pat. No. 6,287,694 (Sep. 11, 2001).    9. J. J. Mack, et al., “Chemical Manufacture of Nanostructured Materials,” U.S. Pat. No. 6,872,330 (Mar. 29, 2005).    10. Morrison, et al., “Forms of Transition Metal Dichalcogenides,” U.S. Pat. No. 4,822,590 (Apr. 18, 1989).
One common feature of these methods is the utilization of liquid or solution-based chemicals to intercalate graphite or other inorganic flake particles. These chemicals often comprise strong acids (e.g., sulfuric or nitric acids), solvents, or other undesirable species that can reside in the material. For instance, Mack, et al. [Ref 9] intercalated laminar materials with alkali metals (e.g. Li, Na, K, Rb, Cs), alkaline earth metals (e.g. Mg, Ca, Sr, Ba), Eu, Yb, or Ti. Intercalation of these elements was accomplished by five different routes: (1) intercalated electrochemically using a non-aqueous solvent; (2) using an alkali plus naphthalene or benzophenone along with a non-aqueous solvent (usually an ether such as tetrahydrofuran); (3) using amalgams (metal+mercury); (4) dissolving any of the afore-mentioned metals in a liquid ammonia solution to create solvated ions; and (5) using n-butyl lithium in a hydrocarbon solvent (e.g., hexane).
In addition to the utilization of undesirable chemicals, in most of these methods of graphite intercalation and exfoliation, a tedious washing step is required, which produces contaminated waste water that requires costly disposal steps. Furthermore, conventional exfoliation methods normally involve a very high furnace temperature (typically between 500° C. and 2,500° C.) since the process depends on vaporization or decomposition of a liquid or solid intercalant. Intercalation with an alkali or alkaline earth metal normally entails immersing the layered material in a metal compound solution (rather than pure metal), allowing the metal ions to penetrate into the inter-layer galleries (interstitial spaces). Typically, metal ion content is relatively low compared to other elements in such a compound solution (e.g., in a solution of 20% by weight lithium chloride in water, lithium content is only 3.27% by weight). Hence, only a small amount of ions from a relatively dilute solution penetrates and stays sporadically in these spaces. The resulting exfoliated product often exhibits platelets of widely varying thicknesses and many incompletely delaminated layers.
In a co-pending application [Bor Z. Jang, Aruna Zhamu, and Jiusheng Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent pending, Ser. No. 11/509,424 (Aug. 25, 2006), now abandoned], we provided an environmentally benign process for exfoliating a laminar or layered compound or element, such as graphite, graphite oxide, and transition metal dichalcogenides, without using undesirable intercalating chemicals. This was a relatively low-temperature process and it produced nano-scaled platelets with relatively uniform thicknesses. This process comprises: (a) subjecting a layered material to a gaseous environment at a first temperature and first pressure sufficient to cause gas species to penetrate into the interstitial space between layers of the layered material, forming a gas-intercalated layered material; and (b) subjecting the gas-intercalated layered material to a second pressure, or a second pressure and a second temperature, allowing gas species to greatly pressurize the interstitial space and thereby exfoliating the layered material to produce partially delaminated or totally separated platelets. In a preferred mode, step (a) of subjecting a layered material to a gaseous environment comprises placing the material in a sealed vessel containing a pressurized gas and step (b) comprises opening the vessel to partially or totally release the gas. Upon pressure release, the material is placed in a pre-heated furnace at a second temperature (which is typically higher than the first temperature) to help soften the intercalated material and instantaneously increase the internal pressure of the interstitial space. This earlier application did not address the issue of mass production. In the present application, we describe, in detail, a specific process that is capable of producing nano-scaled platelets on a semi-continuous basis. This specific process typically involves a higher first temperature and a lower second temperature with a much higher first pressure and a lower second pressure.